The objectives of this proposal are to determine the topographical alterations in protein, free-sterol, and phospholipid distributions which occur in the guinea pig sperm plasma membrane during capacitation (an early phase preparing sperm to participate in fertilization); activation (manifest by a change in the speed and wave of flagellar motion); the acrosome reaction (an exocytotic event releasing spermatozoan enzymes for penetrating egg investments); and gamete fusion in vitro and in vivo. The morphology of sperm membranes in each functional state will be established by electronmicroscopy of rotary-shadowed, freeze-fracture replicas of unfixed, non-cryoprotected cells preserved only by rapid-freezing. Proteins will be identified as intramembranous particles; 3-beta-hydroxysterols will be detected by the forms of complexes they produce with filipin (a polyene) and other sterol-binding reagents; phospholipids will be defined in some instances by the protuberances they exhibit when combined with specific lipid-binding agents, in others by the sites of membrane defects following hydrolysis with lipases. True membrane surfaces will be examined after deep etching. Another major part of the project will concern the isolation and characterization of proteins from the particles of the zipper, a longitudinal array of particles found in the plasma membrane of the principal piece in all mammalian sperm. Eventually, we will test the hypothesis whether selective interference with the interactions of these structurally detectable membrane constituents hinders the ability of sperm to fuse with an egg. Our long-term goal is reversible fertility control.